This invention relates to a filtering method and an apparatus to filter solid impurities from fluids.
One of the most common types of filters is the bed filter. Such a filter may be used in gravity flow or pressurized systems. In a gravity flow bed filter, a fluid (usually a liquid) is fed to the upper surface of the filter media and the flow of the liquid through the bed is promoted by gravity. As the liquid flows through the filter media, suspended solids are trapped and precluded from passing through the media, thus filtering and purifying the liquid. In a pressure system, the fluid (either liquid or gas) is forced to pass through the filter because of the pressure exerted on the fluid. In a pressure system, the fluid may flow in any direction; upward, downward or in a generally horizontal direction. Consequently, the pressurized system is not limited to the downward flow as is the gravity filter. Otherwise, a pressure filter works in a manner almost identical to that of the gravity filter. The fluid flows through a filter media, where the suspended solids are trapped, while the fluid passes through the media. Pressure filters may be built more compactly to accommodate a given flow rate of fluid. Another difference is the fact that both gases and liquids may be filtered in a pressure filter, while only liquids may be filtered in a gravity filter.
With both types of filters, as fluids are being filtered, a layer of the filtered material (precipitate) accumulates on the filter media. This precipitate continues to build up, causing an increasingly greater pressure drop across the filter. After a period of time, the pressure drop becomes unacceptably high. Generally, the high pressure-drop problem is solved by employing a process known as "backwashing", i.e., the flow of the fluid through the filter is reversed. Such a reversal causes a large portion of the precipitate to be removed from the surface of the filter media, thus reducing the pressure drop across the filter. If the backwashed material is again run through the filter, the precipitate immediately builds up on the filter surface again and the pressure-drop across the filter rapidly increases to about the original unacceptable level. To avoid this rapid pressure increase, the backwashed material is usually removed from the filtering system.